The world's most valuable individual prize is not being awarded for a second year because no-one is deemed worthy of winning it.

Sudanese businessman Mo Ibrahim set up the $5m (£3m) prize for African ex-leaders to encourage good governance on the continent.

"The standards set for the prize-winner are high," he said, adding that no new candidates had emerged since last year.

Winners must have been democratically elected and agreed to leave office.

South Africa's Thabo Mbeki, Nigeria's Olusegun Obasanjo and Ghana's John Kufuor are among those who qualified for last year's prize after stepping down in the previous three years.

"It is likely that there will be years when no prize is awarded. In the current year, no new candidates emerged," said a statement from the Mo Ibrahim Foundation.

The winners receive $5m over 10 years, and then $200,000 a year for life.

Mr Ibrahim argues that the prize is needed because many leaders of sub-Saharan African countries come from poor backgrounds and are tempted to hang on to power for fear that poverty is what awaits them when they give up the levers of power.

Botswana's former President Festus Mogae won the prize in 2008 after two terms at the helm of one of Africa's least corrupt and most prosperous nations.

The inaugural prize was given to Joaquim Chissano, Mozambique's former president, who has since acted as a mediator in several African disputes.